Data communication devices (DCDs) are used for data communications between terminals or personal computers over a public switched telephone network (PSTN), a cellular network, a private line or other communications network. Because DCDs are used with portable data terminals having battery power, DCDs may have a low power mode to conserve energy. DCDs in such a low power mode are also referred to as being in a sleep mode. A DCD in the sleep mode must be reactivated or otherwise placed in an operational mode to transmit and receive data. When the DCD is in the operational mode it may be performing a variety of data communication functions, such as configuring the DCD, training an equalizer, arranging data in a format for transmission, forming a data transmission signal, receiving a data transmission signal, and other functions. In order for the DCD to reactivate or transition from the sleep mode to the operational mode, a wake up apparatus or a wake up method is required.
Most DCDs are used in a home or an office environment where power is available from an electrical outlet. In recent years, however, the number of portable terminals, such as notebook computers having a battery for power, has greatly increased. Because the amount of energy that can be stored in a battery is limited and many DCDs receive battery power, these DCDs may have a sleep mode for energy conservation. DCDs may be put in a sleep mode manually or automatically, by removing some or all of the power from the DCD. Manually removing the power is inconvenient and may not work well if a user forgets to instruct the DCD to transition to the sleep mode. After the DCD is placed in a sleep mode, it is necessary to reactivate (wake up) the DCD to an operational mode in order to transfer data. Existing wake up methods either require a waiting period before commands and data can be sent from the terminal to the DCD, or are sufficiently slow that they are not transparent to the user except at very slow baud rates.
A signaling method disclosed in U.S. Pat. No. 4,951,309 describes a power down means that includes transmitting a power down signal from the terminal to the DCD. The power down signal must be removed from the DCD before it can return to the operational mode. The signaling method is not transparent to the user and requires modification of communications software and hardware.
Current wake up methods, moreover, are too slow for transparent autobauding at all of the various terminal baud rates used for terminal-to-DCD data transfer. In addition, the methods used to transition from the sleep mode to the operational mode may require modifications in software, hardware and other equipment, and nonetheless may still require additional, non-transparent operations by the terminal user. Because of these and other problems with the existing wake up methods for DCDs, better methods and devices are highly desirable.